Analysis of nonlinear behavior of power HBTs

To accurately understand the linear characteristics of a heterojunction bipolar transistor (HBT), we developed an analytical nonlinear HBT model using Volterra-series analysis. The model considers four nonlinear components: r(pi), C-diff, C-dep1, and g(m). It shows that nonlinearities of r(pi) and C-diff are almost completely canceled by g(m) nonlinearity at all frequencies. The residual g, nonlinearity is highly degenerated by input circuit impedances. Therefore, r(pi), C-diff, C-dep1, and g(m) nonlinearities generate less harmonics than C-bc nonlinearity. If C-bc is linearized, g(m) is the main nonlinear source of HBT, and C-dep1 becomes very important at a high frequency. The degeneration resistor RE is more effective than R-B for reducing g(m) nonlinearity. This analysis also shows the dependency of the third-order intermodulation (IM3) on the terminations of the source second harmonic impedances. The IM3 of HBT is significantly reduced by setting the second harmonic impedances of Z(S), 2omega(2) = 0 and Z(S), omega(2)-omega(1) = 0.